A: First weight any of the 9 apples.
B: Then he cut all apples in 9 pieces. Put 1 peace of the first apple, 2 peaces of the second apple, 3 peaces of the third, etc... a total of 1+2+...+9 = 45 pieces => y gramms.

From the weight A, we can have the weigh of one piece of apple (one ninth). Let's use a unit 1 to design this weight.

If all apples where the same weight, the weight B should give 45.

Case One : the B weight is lighter than 45 => the first apple is the big apple
Case two : the B weight is heavier than 45.
If the weight is 46 (45+1) the first apple was the heaviest.
If the weight is 47 (45+2) the second apple was the heaviest.
...